[5] The impact of the Long Depression limited government funds available for railway construction, and no extension of the line occurred for over a decade.

Some of the funds for the construction of the Ngākawau-Mokihinui section and the purchase of the Mokihinui line were provided by the Westport Harbour Board.

After the Westport Section was linked to the main part of the South Island railway network in 1943 by the construction of a line through the Buller Gorge, it became known as the Seddonville Branch.

However, as road transport became more prevalent, local businesses abandoned rail cartage and coal was virtually the only freight carried by the late 1930s.

Coal tonnages were declining by this stage: in 1940, the branch was carrying just over half its pre-World War I peak of 800,000 tons.

An early 1967 timetable had one train to Seddonville and the Mokihinui Mine and two to Ngākawau on weekdays, with shuttles from the Conns Creek Branch that diverged at Waimangaroa.

[1] Low demand for Buller region coal, decreased output and a decline in coastal shipping to Westport meant that the remainder carried reduced tonnages.

[6] In the 1980s, traffic rose significantly despite closure beyond Ngākawau and trains ran across the South Island via the Midland Line to the deepwater harbour at Lyttelton rather than to Westport.

Due to the boom in traffic, bogie coal wagons were built to replace ageing four-wheeled stock of much lower capacity.

[6] The WW class were limited on the steep 1 in 33 gradient beyond Seddonville to the Mokihinui Mine, capable of hauling only 180 tons.

The platform edge of Seddonville station remains, but the line to the Mokihinui Mine is now largely inaccessible as it is not near any roads.

[23] At the end of their working lives, some locomotives and rolling stock were used to protect the branch from river erosion.

Between 1958 and 1960, WB class tank locomotives 292 and 299 were dumped on the bank of the Mōkihinui River to ensure the stability of the formation between Seddonville and Mokihinui Mine.